1. Field of the Invention
The present invention relates to a waste treatment system wherein fluidized bed particle scrubbing technology is used for high pressure treatment of wastes. More particularly, the present invention relates to high pressure treatment of wastes without the addition of excess oxygen and without interference by corrosive deposits.
2. Background of the Invention
The oxidation of organic materials to carbon dioxide and water is a combustion process often used to dispose of waste materials in boilers and incinerators to generate useful energy for heating and power generation. In the conventional generation of steam, organic materials are oxidized rapidly in combustion to produce heat that is transferred through a heat transfer surface to pressurized water. Twenty percent of the heating value of the fuel may be lost in the exhaust stack of conventional steam boilers. Over fifty percent of the ability of the fuel to perform useful work, such as for electric power generation, is lost in conventional low pressure steam boilers. The ability of the fuel to perform useful work is referred to as availability or exergy. Exergy has the formula e.sub.x = h.sub.1 - h.sub.2 - T.sub.0 (s.sub.1 - s.sub.2), wherein h.sub.1 and h.sub.2 are the enthalpy of a stream entering and leaving a control surface respectively, s.sub.1 and s.sub.2 are the corresponding entropies, and T.sub.0 is the absolute ambient temperature. Exergy is an analytical tool for optimizing energy.
Exergy analysis indicates that exergy loss is proportional to the heat transferred and the temperature difference across the heat transfer surface. Exergy loss is also caused by excess air and water in the fuel that must be evaporated during combustion at atmospheric pressure.
Heat transfer through surfaces of conventional steam boilers causes loss of availability and often requires the use of specialized materials for exposure to high flame temperatures. On the flame or hot gas side of the tubes, ash or other deposits often impede heat flow and reduce heat transfer Hot spots due to deposition on boiler tubes can cause expensive downtime due to rupture of the tube wall. Heat recovery boilers designed for refuse incinerators are typically lower in electrical output in waste-to-energy cogeneration systems for production of heat and power.
Combustion processes are characterized by emission of inorganic materials in the flue gases as fine particulate, fly ash, and acid gas pollutants, such as SO.sub.2, HCl, CO.sub.2 and nitrous oxides (NOX). These "greenhouse gases" can contribute to flue gases using modern pollution controls, such as dry scrubbing, selective catalytic reduction (SCR), and Benfield processes, known in the art today. Capital costs are high because the acid gases are diluted by other products of combustion, including nitrogen and water vapor.
Products of incomplete combustion (PICS), such as CO and hydrocarbons, dioxins, and furans, require a combination of excess oxygen, time, temperature, and turbulence to minimize air quality impacts. Two-stage combustion has been used to help control emissions of NOX and PICS, such as controlled-air incinerators. A quantity of air below that required for complete combustion is used to pyrolyze the fuel to produce combustible gases that are subsequently burned with excess secondary air. This method, however, still does not assure compliance with air quality control regulations.
Fluidized bed combustion has been developed to reduce acid gas emissions through the use of absorbents such as limestone within the bed. Another benefit of fluidized bed combustion is enhanced heat transfer to external boiler tube surfaces caused by impingement of solids, where erosion of the tube surface can be minimized by proper design. However, with the fluidized bed combustion used in the prior art, the inside surfaces of the boiler tubes are not protected from fouling and corrosion. Only the outer surfaces are protected.
Additionally, in conventional boilers, boiler feedwater is filtered and cleaned to extremely low levels of contaminants to avoid deposition and corrosion of internal surfaces of the boiler tubes.
More stringent regulations have tightened limits on pollutants in the flue gas and water pollutants carried out of the boiler feedwater treatment system or in the blowdown that is used to remove contaminants from a boiler. Ash that is produced from the combustion or gasification of dirty fuels is conventionally separated into fly ash and bottom ash that can require separate testing and disposal using expensive ash treatment and /or expensive specialized lined and monitored landfills.
A gas turbine for burning clean fuels can be combined with a heat recovery boiler and steam turbine in a combined cycle system. Pressurized combustion and lower temperature differences across heat transfer surfaces help to minimize exergy losses and fireside boiler problems. A gas turbine is used to drive a generator and a compressor that supplies air to the combustion chamber. Air is typically supplied at over twice the stoichiometric flow rate necessary for complete combustion of the fuel to control turbine inlet temperature. Compression of this excess air leads to exergy loss.
Capacity of the system is limited by compressor inlet pressure at altitude. For example, a typical gas turbine compressor produces about 25% less flow and about 25% less pressure at 7,000 ft elevation than at sea level. The turbine produces more useful work when operated at a turbine inlet temperature above about 2,000.degree. F., but this tends to produce NOX. Steam or water injection is sometimes used to control NOX formation to below about 50 parts per million by volume (ppmv). Steam injection that is required for NOX control has been found to increase efficiency and the output of useful work from a gas turbine if fuel is added to achieve the same turbine inlet temperature such as in the dual fluid cycle.
Corrosion, erosion, and deposition are problems in steam turbines that are conventionally used to produce useful work from untreated water by isentropic expansion, such as in geothermal energy recovery. Turbine inefficiencies and nonideal gas behavior in isentropic expansion generally cause a loss of useful work in these systems. Gas turbines have the same problems when using dirty fuels at high temperatures that may contain chlorine, sulfur, and alkali metals. Modern materials such as superalloys and resistant coatings have helped to solve these problems in petrochemical and oil refining applications at temperatures up to about 760.degree. C. (1,400.degree. F.), such as in fluid cat cracker power recovery.
In one known process for treating waste organic materials, i.e. the wet air oxidation process described in U.S. Pat. No. 4,229,296, an organic feed and oxidizing agent are pressurized to reaction condition of from about 800 to 2,200 psig, heated to operating temperature and fed to a reactor for residence times of 30 min. to 1 hour. This process is known to be effective for removing 70-95% of the initial organic material. This system is effective for organic waste treatment but has certain disadvantages. Air compression consumes exergy and increases equipment. Often the solubility of oxygen or air in water is below the level required for complete oxidation of the organic materials. Thus, a two-phase water-gas mixture is often used in staged reactors, necessitating provisions for agitation in the reactor to avoid excessive mass transfer resistance between the phases. Often volatile organics such as acetic acid remain after complete processing. The energy available for recovery in the effluent requires separation of liquids and gasses. This technology is a net consumer of energy, as determined by Stone & Stone Webster in DOE/ID-127111-1 (September 1989).
It has long been known that an increase in the speed and efficiency of oxidation of organic substances can be induced by subjecting the substances and oxygen to greatly increased pressure and temperature conditions. Hydraulic columns have been used to create the desired pressure conditions, as described in U.S. Pat. No. 4,594,164 and 4,564,458, up to, and including, the supercritical conditions of water. The critical point is the point of temperature and pressure at which the phase barrier between water and vapor no longer exists. This condition begins to occur at a pressure of 3207 psi and 706.degree. F. Gasses, such as oxygen are fully miscible in all proportions and most inorganic salts are virtually insoluble. Under these conditions, water becomes an excellent solvent for organic substances. These organic columns can reach this pressure added at the location of critical pressure to simultaneously reach critical temperature.
It is taught in U.S. Pat. No. 4,594,164 that the required heat for starting to treat waste within the hydraulic column can be added by burning a supplementary fuel such as propane with oxygen that is compressed for introduction to the critical zone of the column or by preheating all of the fluids in the reactor. Thereafter the reaction can be self-sustaining by transferring the heat of oxidation of organic material to the waste feed stream. In U.S. Pat. No. 4,564,458, it is taught that the required heat can be generated by resistive heating using direct current electrical energy. Unfortunately, this wastes exergy.
Gasses can come out of solution as the effluent rises within the hydraulic column, causing two-phase flow that can induce "geysering" and surges that are difficult to control. The inorganic salts that become insoluble at supercritical conditions in hydraulic columns can dissolve as the effluent rises and pressure is reduced. This stream may require subsequent treatment to remove these salts for recovery of clean water or disposal in accordance with the Clean Water Act. Carbonates can cause scaling problems that inhibit heat transfer and require nitric acid wash for removal.
Hydraulic columns reduce the energy required for pumping the waste by lowering injection pressure to the pressure required to overcome pressure drop in the apparatus and to provide pressurized effluent. Heat is usually transferred from the effluent stream to the incoming waste stream, producing a low temperature effluent stream. The energy available for recovery is generally very small in the effluent from hydraulic column treatment of wastes, thus making this method a net consumer of energy.
Supercritical conditions can be achieved in aboveground apparatus, as proposed in U.S. Pat. Nos. 4,338,199 and 4,453,190. A key feature is that a single fluid phase reaction occurs in the oxidizer at supercritical conditions of the reaction mixture and preferably at the near critical condition of water. Excess oxygen is added to drive the oxidation reactions to completion quickly, thus providing shorter residence time. However, excess oxygen requires the use of specialized materials to avoid excessive corrosion of reactor surfaces.
Several groups have been investigating destruction of hazardous wastes in supercritical water. Because of the high solubility of organics and the low solubility of inorganics in supercritical water, hazardous wastes can be treated and inorganic contaminants removed upon treatment with supercritical water. Most applications have relied upon introducing oxygen to the supercritical reactor to oxidize the organics and to provide the heat required to reach supercritical conditions (374.degree. C., 221 bar). However, several problems have developed from the use of excess oxygen in the system. For example, the excess oxygen causes severe corrosion and requires high energy costs for compression. Additionally, the inorganics are removed as a brine slurry that requires supplemental treatment for discharge. Further, the processing of wastes in supercritical water results in problems of corrosive deposits on the reactor walls.
High pressure boilers, such as are conventionally used in the supercritical steam cycle, produce vapor at about 3,500 psig and 1,000.degree. F. that can produce more useful work. However, saturation is reached by one turbine-generator stage, requiring reheat in the combustion zone of the boiler for additional production of useful work. A conventional supercritical steam plant may have two reheat stages and has a thermal efficiency (to electric power) of about forty percent.
In U.S. Pat. No. 4,543,190, the mixture is reacted in a single fluid phase in a well-insulated reactor to cause the organic material to be oxidized whereby the effluent stream is heated by the oxidation reactions. The heated effluent can be used to provide heat to the reaction mixture through a heat exchange wall surface. Direct mixing is preferred since it enables reaching 706.degree. F. or higher rapidly, i.e., substantially instantaneously, thus avoiding char-forming polymerization that can interfere with heat transfer to the incoming waste stream or incoming oxygen/water mixture.
When acid anions are formed by oxidation of waste materials, the anions can be reacted with appropriate cations, such as sodium, potassium, magnesium, iron or calcium ions, and the resulting inorganic salts are precipitated under reaction conditions. The cations can be provided from their carbonates, hydroxides or oxides. However, carbonate formation interferes with the anion/cation reactions. For this reason, NaOH has been used to react with HCL produced by supercritical water oxidation of chlorinated hydrocarbons to produce NaCl for removal.
The oxidizer can be designed as in U.S. Pat. No. 4,822,497 to allow solids and inorganics that precipitate to be separated into a cooling zone for temperature reduction below critical temperature, this causing the precipitate to dissolve and be readily removed in a brine or slurry for blowdown. However, this stream may require subsequent treatment to remove these salts and solids for production of clean water or disposal in accordance with the Clean Water Act.
Net energy output is restricted by the power requirement of the oxidant compressor that delivers excess oxygen at over 3,207 psia to fully oxidize the organic material and to minimize carbon monoxide in the effluent. The net energy output is further restricted by high power input to the feed pump that delivers sufficient overpressure to power the recycle pump. These processes are net consumers of energy, as determined by Stone & Webster in DOE/ID-12711-1 (September 1989).
It has been suggested that toxic organic materials can be gasified or reformed at the supercritical conditions of water to harmless lower molecular weight materials by breakdown of organic chains and the like whereby the resulting non-toxic materials can be disposed. U.S. Pat. No. 4,113,446 proposes that solid and liquid organic materials can be converted to high BTU gas with little or no undesirable char formation by reaction with water at or above its critical conditions, including the addition of catalysts. However, the conversion to char or coke is undesirably increased as the reaction temperature approaches the critical temperature of water for certain liquid and solid organic materials, regardless of pressure. This char or coke interferes with the heat transfer required to raise the reaction mixture to supercritical conditions.
U.S. Pat. No. 4,113,446 claims an unexpected conversion to high BTU gas occurs above the critical temperature and pressure of water, and that endothermic steam-carbon and exothermic methanation reactions occur concomitantly, so that "very little" heat need be added to the process from external sources. However, the experimental data in U.S. Pat. No. 4,113,446 does not support this claim. Examples in support of this are herein presented.
Two examples are provided which illustrate that addition of "very little" heat as disclosed in Modell is not supported. In a computer simulation, methane and water gas shift equilibrium was assumed. What was found was that significant heat is predicted for devolatilization of organics and heating the water that is not provided by heat of formation of methane. The endothermic heat for devolatilization is less than the heat required to bring water to supercritical conditions. The results of the computer simulation showing that Modell has no support follow.
In the examples, each number represents a particular stream within the simulated waste treatment cycle. The simulated cycle is illustrated in FIG. 1.